Currently, thin wires (bonding wires) having a wire diameter of 20 to 50 μm or so are popularly used as bonding wires for connecting an electrode on a semiconductor element and an external terminal together. A thermal compressive bonding technique with the aid of ultrasound is generally applied to bond bonding wires, and a general-purpose bonding device, and a capillary jig which allows a bonding wire to pass through the interior thereof for connection are used. A leading end of a bonding wire is heated and melted by arc heat inputting, a ball is formed by surface tension, and then the ball is compressively bonded on an electrode of a semiconductor element heated within a range from 150 to 300° C. beforehand. Thereafter, the bonding wire is directly bonded to an external lead by ultrasound compressive bonding.
Recently, techniques related to the structure, material and connection for the semiconductor packaging technologies are rapidly diversified, and for example, in a packaging structure technology, in addition to currently-used QFP (Quad Flat Packaging) using a lead frame, new configurations, such as BGA (Ball Grid Array) using a substrate, a polyimide tape or the like and CSP (Chip Scale Packaging) are practically used, and a bonding wire which has improved loop characteristic, bonding property, mass productivity, usability and the like becomes requisite. Among those improved bonding wire connecting techniques, the wedge-to-wedge bonding technique suitable for fine pitch connection, other than the currently mainstream ball/wedge bonding techniques, requires the fine wire bondability to be improved as the wire is directly bonded at two locations.
Materials to be bonded with the bonding wire have also become diversified. In addition to the conventional Al alloy, copper suitable for a finer wiring has been practically used as a wiring and electrode material on silicon substrates. Ag-plating and Pd-plating are applied onto an upper portion of a lead frame, while copper wirings are applied onto upper portions of a resin substrate, a tape or the like, onto which a film of a noble-metal element such as gold or its alloy is often applied. Depending on a variety of these bonding target materials, a bonding wire is required to be improved in bondability and reliability.
So far, 4N-group gold having a high purity (purity >99.99 mass %) has been mainly used as a material of a bonding wire. Gold is, however, expensive and hence a bonding wire made of another metal material which is less expensive has been desired.
According to the requirements of the wire bonding techniques, it is important to form a ball with a good sphericity at the time of forming the ball and to obtain a sufficient bonding strength in a bonded portion between the ball and an electrode. Further, to cope with lowering of a bonding temperature, thinning of a bonding wire, etc., a bonding strength, a tensile strength and the like are requisite at a part where a bonding wire is subjected to wedge bonding to a wiring on a circuit wiring board.
In the resin encapsulation process of injecting high-viscosity thermosetting epoxy resin at high speeds, there occurs a problem that a bonding wire is deformed to come in contact with the adjacent wires, and besides the wire deformation is required to be restrained as much as possible, in a resin encapsulation process under the situations where pitches become finer, wires become longer and thinner. Although such deformation can be controlled to some extent by an increase in wire strength, this resin encapsulation process still involves difficulties in putting it for practical application, unless some problems that loop control becomes difficult and the strength in bonding decreases, etc. are solved.
Besides, long-term reliability is also important at the time of actual use of a semiconductor element on which a bonding wire is connected and mounted. Particularly for a semiconductor element or the like mounted on an automobile, high reliability under such severe environments as a high temperature, high humidity, heat cycle or the like is required in order to ensure strict safety. Even in such unprecedented severe environments, high reliability must be maintained without deteriorating a bonded portion where the bonding wire has been connected.
As wire characteristics for satisfying the above requirements, it is desired that overall characteristic features thereof be satisfied, such as easy loop control in a bonding process, improved bondability to an electrode and a lead wire, restrained excessive wire deformation in a resin encapsulation process subsequent to the bonding process, and furthermore, long-term reliability in a connected portion as well as stability at a bonded portion under severe environments.
A bonding wire made of copper has been developed in order to achieve low material cost, excellent electric conductivity, enhanced ball bonding and wedge bonding properties, as is disclosed in patent literature 1, etc. According to the copper bonding wire, however, there occur problems that the oxidization of the wire surface reduces bonding strength and the wire surface is prone to generate corrosion or the like at the time of resin encapsulation. These problems are partially responsible for the lack of progress in practical application of the copper bonding wire.
In the copper-based bonding wire, when forming a ball by melting a wire tip, the bonding process is performed with a gas sprayed onto the wire tip in order to inhibit oxidation. Currently, a nitrogen gas containing 5 vol % hydrogen is generally employed as an atmospheric gas used in forming a ball of the copper-based bonding wire. In patent literature 2, it is disclosed that when a copper wire is bonded to a copper lead frame or a copper alloy lead frame, the bonding process is performed in the atmosphere containing 5 vol % H2+N2. Also, in non-patent literature 1 is reported that in forming a ball of the copper bonding wire, the 5 vol % H2+N2 gas can prevent a ball surface from being oxidized and therefore, the 5 vol % H2+N2 gas is more desirable than a N2 gas. Today, the 5 vol % H2+N2 gas has been standardized as a gas used in employing the copper-based bonding wire.
As a technique of suppressing any oxidization of a surface of a copper bonding wire, patent literature 3 discloses a bonding wire in which copper is covered with a noble metal or a corrosion-resistant metal, such as gold, silver, platinum, palladium, nickel, cobalt, chrome, titanium, and the like. Moreover, from the standpoint of a ball formability and suppression of deterioration of a plating solution, patent literature 4 discloses a bonding wire so structured as to have a core member mainly composed of copper, a dissimilar metal layer formed on the core member and made of a metal other than copper, and a coating layer formed on the dissimilar metal layer and made of an oxidization-resistant metal having a higher melting point than copper. Patent literature 5 discloses a bonding wire comprising a core member mainly composed of copper, and an outer skin layer which contains a metal, having either one of or both of a constituent and a texture different from the core member, and copper, and which is a thin film having a thickness of 0.001 to 0.02 μm.    Patent literature 1: Japanese unexamined patent application publication No. S16-99645    Patent literature 2: Japanese unexamined patent application publication No. S63-24660    Patent literature 3: Japanese unexamined patent application publication No. S62-97360    Patent literature 4: Japanese unexamined patent application publication No. 2004-64033    Patent literature 5: Japanese unexamined patent application publication No. 2007-12776    Non-patent literature: “Copper Ball Bonding for Fine Pitch, High I/O Devices”: P. Devlin, Lee Levine, 38th International Symposium on Microelectronics (2005), P.320-324.